UNIVERSITY  OF  CALIFORNIA      AGRICULTURAL    EXPERIMENT    STATION 
COLLEGE    OF    AGRICULTURE  benj.  ide  wheeler,  president 

THOMAS    FORSYTH    HUNT.    Dean  and  Director 

BERKELEY 


CIRCULAR  No.  102 
(June,  1913) 

THE  WOOLLY  APHIS 


BY 

C.  W.  WOOD  WORTH 


The  best  known  of  the  insects  affecting  the  apple  is  called  the 
woolly  aphis  because  of  the  great  amount  of  wax  secreted  by  the 
aphid,  making  conspicuous  white  woolly  masses  on  the  under  sides 
of  the  twigs,  in  patches  on  the  trunk  and  larger  branches,  and  even 
on  the  leaves  or  within  the  core  of  bell-flowers.  The  same  insect 
occurs  also  on  the  roots  but  does  not  produce  as  abundant  wax  in  this 
situation.  The  presence  of  the  insect  results  in  swellings,  usually  of 
small  size  and  characteristic  appearance. 

LIFE  HISTORY 

Recent  studies  in  the  Maine  Experiment  Station  have  proven  that 
the  insect  is  only  another  form  of  the  elm  aphis,  Eriosoma  ulmi,  and 
this  becomes  the  technical  name  of  the  woolly  aphis. 


Curled  leaves  on  elm  produced  by  the  woolly  aphis. 


On  the  Elm. — The  history  of  the  insect  on  the  elm  is,  that  in  the 
fall  a  winged  insect  from  some  source  previously  unknown,  arrives 
on  the  elm  and  gives  birth  to  peculiar  wingless  lice  of  both  sexes, 
incapable  of  feeding,  which  wander  over  the  bark,  the  female  entering 
a  crevice  and  laying  a  single  egg  almost  as  large  as  herself.  This 
egg  hatches  about  the  time  the  first  leaves  appear  in  the  spring. 

The  louse  hatched  from  the  winter  egg  locates  on  the  under  side 
of  a  small  leaf  and  proceeds  to  found  a  colony.  It  is  only  a  few  days 
before  there  are  grand-daughters  and  great  grand-daughters.  In  the 
meantime  the  leaf  has  begun  to  curl  so  as  to  produce  a  great  pocket 
in  which  the  colony  continues  to  increase  until  there  are  thousands 
of  individuals,  all  wingless  females,  producing  living  young.  Finally, 
as  the  leaf  matures,  the  young  produced  develop  into  winged  insects 
that  fly  away  and  the  older  members  of  the  colony  die,  so  that  before 
midsummer  the  large  swollen  leaves  are  without  living  inhabitants 
though  they  cling  to  the  tree  all  summer  and  the  following  winter 
as  well. 

On  the  Apple. — We  now  know  that  these  winged  lice  from  the 
elm  leaves  return  to  the  apple.  The  history  of  the  insect  in  most 
parts  of  California  is  the  development  of  wingless  females  in  uninter- 
rupted succession  in  both  the  roots  and  the  twigs,  both  summer  and 
winter,  with  now  and  then  a  certain  proportion  becoming  wringed  and 
almost  any  time  during  the  summer  or  fall  the  air  may  suddenly  be 
filled  with  these  migrating  winged  insects.  Of  course  in  these  migra- 
tions by  far  the  greater  number  of  individuals  fail  to  find  a  suitable 
place  to  locate  and  simple  die.  There  is  also  to  be  noted  at  times, 
especially  in  the  spring,  definite  wandering  periods  in  the  case  of 
wingless  individuals  whereby  the  insects  become  scattered  over  the  tree. 

The  time  of  greatest  increase  is  in  the  spring  when  the  tree  is  also 
growing  vigorously,  before  midsummer  the  colonies  loose  vigor, — indeed 
generally  fail  to  hold  their  own  and  decrease  spontaneously,  the  trees 
becoming  apparently  free,  perhaps  only  a  few  in  some  sheltered  spot 
on  the  trunk,  carrying  the  insect  over  to  the  next  season.  Similar 
changes  in  numbers  are  noticeable  in  the  case  of  the  insects  on  the 
roots  and  in  the  spring  a  definite  migration  of  wingless  insects  from 
the  roots  to  the  twigs  has  been  noticed.  An  egg  laying  form  on  the 
apple  has  been  described  as  occurring  in  the  fall,  but  the  writer  has 
not  observed  it  at  Berkeley  nor  in  the  Pajaro  Valley. 

ECONOMICS 
The  woolly  aphis  is  almost  universally  present  where  apples  are 
grown,  but  is  of  very  unequal  importance.     As  far  as  is  known  it 


3 

is  native  to  Europe  but  is  of  small  enough  significance  that  the  apple 
infesting  form  was  not  described  for  nearly  a  century  after  the  elm 
form  was  made  known.  English  entomologists  believed  that  the 
insect  was  of  American  origin  calling  it  "American  Blight." 

RESISTANT  ROOTS 
In  Australia  it  is  particularly  serious  and  has  been  successfully 
combatted  by  the  use  of  Northern  Spy  roots.  These  roots  have  proven 
equally  effective  in  California  and  are  certainly  to  be  recommended 
wherever  the  wolly  aphis  is  a  serious  pest.  There  is  perhaps  no  part 
of  the  State  where  resistant  roots  are  absolutely  essential  to  the  profit- 
able growing  of  apples. 

CONTROL  IN  THE  NURSERY 

The  economic  problem  in  the  nursery  is  essentially  different  from 
that  in  the  orchard,  because  horticultural  inspectors  generally  insist 
on  stock  being  entirely  free  from  any  sign  of  the  presence  of  the 
aphid.  Nursery  stock  grown  where  the  aphid  is  not  serious  enough 
to  prevent  normal  growth  and  intended  for  planting  in  similar  locali- 
ties is  intrinsically  as  valuable  when  infested  as  when  free,  but, 
because  many  inspectors  do  not  take  this  view,  it  is  essential  for  nur- 
sery men  to  produce  stock  free  from  the  insect  when  intended  for 
general  sale.  This  requires  the  prevention  of  wing  production  in  the 
neighborhood  of  the  nursery.  When  elm  trees  occur  in  the  neighbor- 
hood they  should  be  carefully  inspected  in  the  spring  as  soon  as 
the  swollen  leaves  become  conspicuous  and  these  leaves  removed. 
Nearby  apple  trees  should  be  kept  free  all  summer  from  the  twig 
infestation  by  the  methods  detailed  below.  There  seems  to  be  no 
danger  of  winged  forms  issuing  from  the  roots. 

Should  the  nursery  become  infested  it  is  well  to  dig  up  and  destroy 
everything  that  has  been  attacked  as  soon  as  discovered  as  the  wingless 
forms  may  spread  along  nursery  rows. 

CONTROL  IN  THE  ORCHARD 

The  great  majority  of  orchardists  in  this  State  do  not  consider  the 
woolly  aphis  injury  serious  enough  to  take  any  measures  toward 
suppressing  it.  In  some  localities  every  year  and  elsewhere  during 
some  seasons,  treatment  is  amply  justifiable. 

Oil  Treatment. — The  simplest,  and  in  most  cases,  the  cheapest  treat- 
ment is  the  direct  application  of  kerosene  or  gasolene  by  means  of  a 
brush  or  small  cloth  swab.  Do  not  use  too  much  oil  for  fear  of 
making  dead  spots  on  the  bark.  If  this  method  is  employed  one  should 
begin   early   enough   in  the  spring  while   the   insect   is   in  scattered 


colonies.  The  work  can  be  done  as  quickly  as  spraying  and  is  imme- 
diately and  completely  effectual.  The  trees  should  be  gone  over  about 
once  a  week  during  the  spring  till  the  danger  of  rapid  increase  is  past. 
Spraying  is  necessary  if  the  insects  have  been  allowed  to  become 
too  numerous  to  be  economically  treated  with  oil.  The  best  insecticide 
for  aphids  is  nicotine.    The  following  formula  is  effective : 

Nicotine  sulfate  40  per  cent  lib. 

Cresol  soap  1  gal. 

Water  200  gal. 

1-%    lbs.    when    using    30    per    cent. 

The  manufacturers  recommend  the  use  of  twice  as  much  nicotine. 
The  soap  is  added  simply  to  increase  the  penetration.  Whale  oil  soap 
may  be  substituted,  but  use  ten  times  as  much.  There  are  several 
brands  of  tobacco  extract,  weaker  than  40  per  cent.  If  the  weaker 
nicotine  is  used,  substitute  l1/',  lbs.  when  using  30  per  cent ;  2  lbs.  when 
using  20  per  cent ;  4  lbs.  when  using  10  per  cent ;  10  lbs.  when  using  4 
per  cent;  50  lbs.  if  the  tobacco  stems  are  used  instead  of  the  extract. 
In  this  case  the  stems  should  be  steeped  in  warm  water  to  extract  the 
nicotine  and  then  the  soap  added  to  this  extract.  Spraying  must  be 
very  thoroughly  done  to  give  satisfactory  results. 

Sand  Treatment. — It  may  sometimes  be  desirable  to  prevent  the 
migration  of  the  lice  from  the  roots.  Since  the  insects  cannot  burrow 
through  the  soil  but  must  follow  cracks  and  almost  always  migrate 
along  the  trunk  of  the  tree,  the  easiest  means  of  preventing  their 
movements  is  to  dig  out  a  few  shovels  of  earth  and  fill  in  the  space 
with  sand,  wrhich  does  not  crack. 


